capt_rs,
I seem to recall that in your original post, you asked for the applicable formula. Here it is :-
TAS = 38.975 X Mach No. X SQR (Absolute Temperature)..... (Where Absolute Temperature = SAT + 273.15)
The respondants, whilst providing sound replies, have made a rather simple question into a bleedin' dialogue!!!!!!!!!!!
By swapping around the variables to find what you want, here we go:-
Absolute Temperature = (TAS / 38.975 / Mach No.)^2
BY Substitution :-
Ta = (499 / 38.975 / .84)^2 = 232.31 Degrees Absolute - 273.15 = -40.84 Degrees C
Simple isn't it, and I thought that I was the last of the pedants. The F/L 390 is a "red herring", only necessary if you require the deviation from ISA. (In which case it is ISA + 15.66 C).
Regards,
Old Smokey (Reformed Pedant)